go-ipfs-config: add a Clone function

The user must call this before modifying the config. Given that the config
contains slices/maps modifications can modified *shared* state, even after
dereferencing.

config/config.go

@@ -110,3 +110,19 @@ func ToMap(conf *Config) (map[string]interface{}, error) {
 	}
 	return m, nil
 }
+
+// Clone copies the config. Use when updating.
+func (c *Config) Clone() (*Config, error) {
+	var newConfig Config
+	var buf bytes.Buffer
+
+	if err := json.NewEncoder(&buf).Encode(c); err != nil {
+		return nil, fmt.Errorf("failure to encode config: %s", err)
+	}
+
+	if err := json.NewDecoder(&buf).Decode(&newConfig); err != nil {
+		return nil, fmt.Errorf("failure to decode config: %s", err)
+	}
+
+	return &newConfig, nil
+}

config/config_test.go

@@ -0,0 +1,23 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestClone(t *testing.T) {
+	c := new(Config)
+	c.Identity.PeerID = "faketest"
+	c.API.HTTPHeaders = map[string][]string{"foo": []string{"bar"}}
+
+	newCfg, err := c.Clone()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if newCfg.Identity.PeerID != c.Identity.PeerID {
+		t.Fatal("peer ID not preserved")
+	}
+	delete(c.API.HTTPHeaders, "foo")
+	if newCfg.API.HTTPHeaders["foo"][0] != "bar" {
+		t.Fatal("HTTP headers not preserved")
+	}
+}